1. Field of the Invention
The present invention relates to packet data communication. More particularly, the present invention is concerned with a packet data retransmission control method, and a system, that enjoy high efficiency in retransmission due to efficient reception of an upstream control signal Ack or Nack during high-speed downstream packet data transmission based on the high-speed downlink packet access (HSDPA) technology or the like.
2. Description of the Related Art
With the advent of the asynchronous digital subscriber line (ADSL) technology or the like, high-speed data transmission that is especially fast in transmission of downstream data is prevailing among households. Accordingly, mobile radio communication is requested to be especially fast in transmission of downstream data. For the wideband-code division multiple access (WCDMA) technology that is one of the new-generation portable cellular phone systems (International Mobile Telecommunications (IMT)-2000), HSDPA is likely to be adopted as a standard technology for high-speed downstream data transmission (WCDMA is referred to as 3GPP TR25.308).
FIG. 1 shows an example of a normal retransmission control form in which retransmission control is exchanged between a base station and a mobile station or a terminal station. FIG. 2 shows an example of retransmission of packet data shown in FIG. 1.
Referring to FIG. 1, a base-station upper-level system 3 transfers packet data to a base station 2-1 that accommodates a terminal station 1 which is a data terminating side. The base station 2-1 transfers the received packet data to the terminal station 1. In this case, the terminal station 1 returns an upstream control signal Ack or Nack, with which reception of packet data is acknowledged, to the base station 2-1 that is the data transferring side.
A description will be made by taking the case shown in FIG. 2 for instance. The base station 2-1 transmits packet data 1 received from the base-station upper-level system 3 to the terminal station 1. When the terminal station 1 receives the packet data 1 correctly, the terminal station 1 returns a signal Ack to the base station 2-1. Consequently, the base station 2-1 judges that the packet data 1 has been transmitted correctly, and transmits the next packet data 2 to the terminal station 1. In this case, a data error occurs, and the terminal station 1 returns a signal Nack to the base station 2-1. Consequently, the base station 2-1 judges that the packet data 2 has not been correctly transferred, and retransmits the packet data 2 to the terminal station 1.
Furthermore, in this case, if an acknowledgement response to the packet data 3, which is sent from the base station 2-1, returned from the terminal station 1 is not received or is unidentified, the base station 2-1 judges that the packet data 3 has not been correctly transferred. The base station 2-1 then retransmits the packet data 3 to the terminal station 1. Normal retransmission control is exchanged directly between the base station and the terminal station.
FIG. 3 shows as other example of a retransmission control form an example of a retransmission control form based on a technology other than HSDPA combined with WCDMA.
Herein, a site diversity method is adopted in order to produce a large diversity handover gain, so that a plurality of base stations 2-1 to 2-n will transmit or receive a signal to or from the terminal station 1. Actions to be performed according to the method will be described by taking the same case as that shown in FIG. 2 for instance. Assuming that downstream packet data is transmitted, the base-station upper-level system 3 transfers the same packet data to the base stations 2-1 to 2-n. The base stations 2-1 to 2-n each transmit the same packet data 1 to the terminal station 1.
The terminal station 1 synthesizes all the packet data items 1 received from the respective base stations 2-1 to 2-n or selects any of the packet data items 1. If the received packet data 1 is correct, the terminal station 1 returns a signal Ack to each of the base stations 2-1 to 2-n. If the next packet data 2 cannot be received correctly, the terminal station 1 returns a signal Nack to each of the base stations 2-1 to 2-n. 
Each of the base stations 2-1 to 2-n receives the signal Ack or Nack and transmits the result of reception to the base-station upper-level system 3. The base-station upper-level system 3 judges whether the packet data has been correctly transferred through synthesis of n signals or selection of any of the n signals. If the base-station upper-level system 3 judges that the packet data has been transferred correctly, it transmits the next packet data. If the base-station upper-level system 3 judges that the packet data has not been correctly transferred, it retransmits the same packet data. If neither the signal Ack nor the signal Nack is returned (identified) as a response with which reception of the packet data 3 is acknowledged, that is, if an unidentified signal is returned, the signal Nack is judged to have been returned. The packet data 3 is then retransmitted. Thus, in the retransmission control form based on a technology other than HSDPA combined with WCDMA, retransmission control involves the base-station upper-level system 3 and terminal station 1.
FIG. 4 shows, as another example of a retransmission control form, a retransmission control form based on WCDMA combined with HSDPA.
Even when WCDMA is adopted, if WCDMA is combined with HSDPA, basic retransmission control is, similarly to normal retransmission control in the form shown in FIG. 1, exchanged between the base station 2-1 and the terminal station 1. The retransmission control process is identical to that described in conjunction with FIG. 2. However, in this example, from whichever of the base stations 2-1 to 2-n the terminal station 1 will receive packet data cannot be designated. The base-station upper-level system 3 transmits packet data to the plurality of base stations 2-1 to 2-n.
According to HSDPA, although a diversity handover gain is sacrificed, retransmission control is supposed to be exchanged between the base station and the terminal station. In a retransmission control process based on WCDMA not combined with HSDPA, a gain in reception is guaranteed by the site diversity method. However, when a large amount of data is transferred at a high speed, as a plurality of transmission lines is occupied, a line occupancy rate decreases. Moreover, when a transfer method that modifies a physical configuration, such as, a method of transmitting a different redundant bit at every time of retransmission, a method of modifying a data length to be interleaved, or a method of changing modulation forms (quaternary, octal, hexadecimal, etc.) is adopted for retransmission, WCDMA not combined with HSDPA cannot be adopted.
FIG. 5 lists the aforesaid three conventional retransmission control methods for comparison.
As seen from FIG. 5, according to the normal retransmission control method and the retransmission control method based on WCDMA combined with HSDPA, retransmission control is exchanged between a base station and a terminal station. This is advantageous in terms of a line occupancy rate or a delay time in retransmission. The normal retransmission control method and the retransmission control method based on WCDMA combined with HSDPA are nearly identical to each other except a data transmission rate. On the other hand, according to the retransmission control method based on WCDMA not combined with HSDPA, retransmission control is exchanged between a base-station upper-level system and a terminal station in order to produce a diversity handover gain. Although production of a diversity handover gain is advantageous, this method is disadvantageous in terms of the line occupancy ratio or the delay time in is retransmission.